1. Field of the Invention
The present invention relates to a fully reflective and highly thermoconductive electronic module and a method of manufacturing the same.
2. Related Art
Heat is generated when a light-emitting diode (LED) is operating with electrons flowing therethrough. The generation of heat increases the resistance and affects the flows of the electrons so that the function of the LED is significantly influenced. When the technology of manufacturing the LED is greatly enhanced, a line width in the LED is getting smaller and smaller, and the line density in the LED is getting higher and higher. Thus, the heat generated by the LED is increased rapidly. Taking the high-luminance LED as an example, its thermal density is higher than or equal to 100 W/cm2. Thus, the heat dissipating ability of the substrate contacting with the LED is a key factor for determining whether the LED can operate normally or not.
A typical power component, such as a solid relay, is similar to the CPU of the computer and generates a lot of heat. Thus, the power component also dissipates the heat rapidly through the substrate contacting therewith so that it can operate normally.
At present, the LED has been applied to the lighting. However, the major barrier on the applications of the LED as the light source is that the LED cannot survive at an elevated temperature. Generally speaking, the temperature of the LED cannot exceed 90° C. If the temperature of the LED is higher than 90° C., the luminance thereof rapidly deteriorates. So, the rapid heat dissipating ability of the heat dissipation substrate in contact with the LED has become a greatest challenge for determining whether the LED can become the illumination light source or not. It is widely accepted that the development of the heat dissipating substrate has played an important role on the applications of the LED as the light source.
In order to satisfy the miniaturized requirement of the LED, the substrate contacting with the LED has to satisfy the following fundamental requirements.
First, the material must have a high thermal conductivity to dissipate the heat rapidly.
Second, the material must have the high resistivity in order to prevent the P and N electrodes of the LED from being short-circuited.
Third, the substrate must direct all the light rays emitted from the LED toward the front side of the LED after the above-mentioned conditions are satisfied, such that the effective luminance toward the front side of the LED can be increased.
Recently, various color LEDs have been gradually developed, wherein the successful development of the white-light LED has attracted considerable attention. This is because the white-light LED can serve as a light source for an illumination lamp. One of the bottleneck in the LED illumination technology is the heat dissipating problem. If the heat cannot be rapidly dissipated, the temperature of the LED chip becomes too high, the light emitting efficiency of the LED chip is lowered, and the lifetime of the LED chip is shortened. The LED chip may be mounted on the heat dissipating substrate. The major function of the substrate is to dissipate the heat to the heat dissipating fins or heat pipes.
In addition, because the LED chip has P and N electrodes, the substrate in contact with the LED chip also needs to have the separate lines to connect to the P and N electrodes independently. At present, all the available heat dissipating substrates, such as FR4 and MCPCB substrates, may provide the electroconductive requirement. However, the heat dissipating abilities of the two heat dissipating substrates have the significant difference. For example, the thermal conductivities of the two heat dissipating substrates as measured by the flash method are listed in Table 1.
TABLE 1Heat dissipating substrateThermal conductivity (W/mK)FR4 (Flame Retardant 4)~0.4MCPCB (Metal Core Printed~3Circuit Board)
For all the heat dissipating substrates for the LED chips, the thermal conductivity still must be enhanced to deal with the heat dissipating requirements of the LED chips with the higher power. Furthermore, the problem of light reflection should also be addressed. At the moment, all the available heat dissipating substrates are not able to cope with the optical, electrical and thermal requirements simultaneously.
Electrons and holes in the P-N junction of the LED chip react with each other to release light rays, which travel everywhere due to scattering and reflecting. Thus, only a portion of light rays can travel in the frontward direction of the LED chip and become the useful light source. Thus, when the LED chip or lamp is being packaged, chemical or physical coating often has to be applied to the peripheral surfaces by way of, for example, chemical vapor deposition (CVD), physical vapor deposition (PVD), or the like. So, the reflective metal layer may be coated thereon to increase the effective light intensity. Alternatively, it is possible to apply a metal layer to the heat dissipating substrate and thus to enhance the light reflecting ability of the heat dissipating substrate. For example, Wang et al. disclosed a low temperature co-fired ceramic (LTCC) tape compositions, light emitting diode (LED) modules, lighting devices and a method of forming thereof in U.S. Pat. No. 7,550,319, wherein the ink containing silver and glass is printed on the ceramic substrate by way of screen printing, and then the silver is combined with the ceramic substrate by way of co-firing. Because the silver can reflect the light, the reflecting function may be provided. However, the cost of such technique is very high.